Parametric relaxation in whispering gallery mode exciton-polariton condensates

Abstract

Polariton condensation of one-dimensional multimode whispering gallery mode exciton polaritons is investigated in hexagonal ZnO microwires at cryogenic temperatures. At threshold, stimulated scattering is fed by the resonant emission from highly populated defect-bound excitons. With further increasing excitation power, condensates relax within the multimode whispering gallery mode polariton ladder, leading to their effective evaporative cooling. The relaxation is a parametric polariton-polariton scattering process evidenced by its intensity, energy, and momentum evolution. The experimental observations are in good agreement with numerical simulations based on a model for polariton-polariton scattering extended to the multimodal whispering gallery mode system. [4]. Multimode polariton systems that undergo inter-or intrabranch parametric scattering processes [5][6][7] are promising for sources of entangled photons [8][9][10]. However, using those effects in integrated optical circuits requires manipulability and guidability that is more easily achieved in one-dimensional microcavities. A major step towards this goal has recently been demonstrated in polariton microcavities etched into wire-shaped structures resulting in the formation of one-dimensional polariton condensates with long polariton lifetimes and long propagation lengths [11]. Although parametric oscillation processes have already been demonstrated in those structures [6], a richer mode structure is necessary for the creation of entangled polaritons [8]. Our "bottom up" approach for the creation of one-dimensional multimode polariton systems combines the excellent photonic properties of whispering gallery modes (WGMs) with the outstanding electronic properties of excitons in hexagonal ZnO microwires. Beneficially, ZnO microwires can easily be grown by carbothermal vapor-phase transport [12] and naturally provide strong photonic confinement by total internal reflection (TIR) [13]. Hence, they obviate the need for highly reflecting Bragg mirrors and sophisticated fabrication steps. One-dimensional WGM exciton-polariton condensates in those ZnO microwires then benefit from the multimodal character of the photonic component along with a long photon lifetime. * cpd3@st-andrews.ac.uk † schmidt-grund@physik.uni-leipzig. de Here, we demonstrate the condensation of WGM exciton polaritons at cryogenic temperature being accompanied by complex parametric polariton scattering processes. We observe an efficient exciton relaxation into defect-bound states, whose radiative decay resonantly excites the polariton condensate above threshold in one of the many lower polariton branches. By increasing the excitation power, condensate relaxation into further lower polariton states takes place, providing an efficient evaporative cooling mechanism. We show experimentally and theoretically that this process is caused by multichannel scattering mechanisms leading to parametric mixing of polariton states at higher excitation densities, as evidenced by constant energy and momen...

Interplay between tightly focused excitation and ballistic propagation of polariton condensates in a ZnO microcavity

“…However a discrepancy is observed between the experimental k-space distribution of the polariton condensate and the simulated one; the polaritons don't reach the predicted maximum value of their wavevector far from the excitation spot. Two possible explanations can be proposed that would also require further investigations: (i) at large polariton densities in the condensate, part of the polaritons are ejected from the condensate towards other polariton branches [18,37], leading to a decrease of the polariton lifetime; our simulations suggest that even a fourfold increase of the polariton decay rate 𝛾 𝑝𝑜𝑙 is not enough to accound for the k-space experimental results. (ii) Even if they don't relax in energy, the polaritons within the condensate may relax their wavevector due to polariton-polariton scattering.…”

Polariton condensates at room temperature

“…Even if the order of magnitude of those three processes is very different, this relaxation picture is very similar to the one of GaAs or CdTe microcavities. They provide an efficient scattering mechanism for parametric processes along the polariton branches [49,51]. This process provides an efficient gain band, as studied in the 1970's in II-VI bulk semiconductor lasers [93,94], and in the 2000's in ZnO nanorod lasers [44,[95][96][97].…”

“…Even if the order of magnitude of those three processes is very different, this relaxation picture is very similar to the one of GaAs or CdTe microcavities. They provide an efficient scattering mechanism for parametric processes along the polariton branches [49,51].…”

Interacting plexcitons for designed ultrafast optical nonlinearity in a monolayer semiconductor

“…Currently, the highest polariton nonlinearity is usually achieved in GaAs 12 , 13 and CdTe 14 microcavities with large Bohr radii, but operation at cryogenic temperatures is required due to their small exciton binding energy. Instead, robust room-temperature exciton-photon coupling strength can be realized in ZnO 15 and organic systems 16 with large exciton binding energy (small Bohr radii), but often at the expense of weak polariton nonlinearity. This contradiction severely limits the development of room-temperature polaritonic devices, in which sufficiently strong nonlinearity and coupling strength are simultaneously required.…”